Motor Driver Relay Circuit

Hi all,
I have a serious problem. I am working on a robot. I use small and big motors for various things. I am using a relay circuit for driving motors, that's in addition to the microcontroller of course. I am using separate power supplies for each of the microcontroller and the motor. The microcontroller keeps on reseting !! and its vcc keeps jumping !!
That's in the case of using big motors of high current while in case of using the small motors with the relay circuit, everything is OK !!!

I tried using a buffer and I tried changing the switching transistors but it keeps reseting. Does anyone know what could be the problem ? I am using AT90S8515

if possible use opto isolator. also do you have diodes connected to the relays the "wrong way"? relays are also creating big "sparks" when opened/closed. the diode must be from GND to Vcc cause relay creates those pulses the wrong way and then the spark wont get further than the diode so all is oke. the diode could be 1N4007

Thx for ur reply, what's an optoisolater

opto isolator. optical isolator. a IC that has a Led and a phototransistor in it.

Thanks it worked just fine